The invention relates to improvements in and relating to the field of investment casting. Investment casting is a well known process and is widely used in the production of metallic products. The process of forming an investment casting mould involves the dipping of an expendable pattern, commonly a wax model, into a slurry comprising a refractory material and a binder followed by the model's removal from the slurry. A coat of the slurry remains upon the model. A stucco coating of dry powdered refractory material is applied to the wet slurry coating the model and the slurry coating is then either dried or allowed to dry. After the first slurry coating and stuccoed layer have dried, further layers may be added by repeating the dipping, stuccoing and drying process until a coating of desirable thickness and green strength has been created.
The finished coating, which commonly comprises a number of dried slurry and stucco layers, eventually forms the mould. After a final layer of the coating has been dried, the wax model and coating are heated and, as the wax model melts, it is eliminated from the coating leaving a coating shell. The coating shell is then fired in order to remove any volatile organic residues present and to stabilize the shell. The shell is stabilized by ceramic bonds formed by sintering. The mould formed by the firing of the shell may then be filled with molten metal. The molten metal is allowed to cool and solidify to form a cast conforming to the inside of the mould. The mould is then removed from the metal.
Of course, the slurry may be applied to the wax model by a method other than dipping, such as coating.
Customarily, slurries used in investment casting contain a refractory material and a binder. Environmental considerations dictate that the binder should be water-based rather than say alcohol based. Ethyl silicate has been used as a binder but its expense and associated environmental issues commonly preclude its use. Silica sols are used in binder compositions. Where silica sols are used, the time taken for the drying process is disadvantageously long.
A single stuccoed slurry layer, applied to a wax model in the course of investment casting, may take between 3-8 hours to dry. Where the model comprises recessed parts or other complex features, the drying time may be increased to 24 hours or more. In the production of a shell mould comprising a number of layers, the total drying time is between 12 hours and several days. This significantly increases the cost of the process.
Organic polymers have been incorporated into sol binder compositions in order to improve the drying times of slurry coats formed therewith, for example U.S. Pat. No. 6,020,415 discloses a binder composition comprising a latex polymer.
Soluble organic polymers commonly dissolve and ‘wet out’ of the mould. It has been found that the green strength of moulds produced using such polymers suffers as a result.
Steam permeation through the mould on heating may occur at temperatures of around 200° C., or for example during the elimination of the wax model. This may substantially reduce the green strength of the resulting mould by up to 50%.
The incorporation of fibers into slurries for use in investment casting mould production in order to reduce the cracking of the mould produced, is exemplified in GB 1410634 B in which a thixotropic dipping mixture comprising fibers, a dispersing agent, a refractory material and a binder is disclosed. The dipping mixture is said to reduce or obviate the cracking of the mould during the drying process.
The patent applications GB2350810, U.S. Pat. No. 6,450,243 and the related international patent application WO/2001/068291 disclose methods of investment casting which involve slurries comprising insoluble organic fibers.
During further testing it has become clear that there are problems associated with the binder and slurry compositions of the prior art. It is among the objects of the present invention to solve one or more of the following problems.
It has been noted that insoluble fibers added to a binder composition comprising a silica sol may separate from the binder by floating to the surface or sinking to the bottom of the sol. These fibers may also ball up/cluster. Some of the fibers' effectiveness may therefore be lost.
Foundry slurries for use in investment casting containing materials of widely disparate specific gravities need continual stirring or other agitation in order to remain evenly dispersed. Without such agitation the dense material in the slurry, for example the dense ceramic refractory materials, will separate from the less dense liquid slurry components. Mechanical failure of the agitation means or power failure of the source powering the agitation means can lead to rapid compaction and/or separation of slurry components. This separation will at least cause production delays and at worst cause the loss of a considerable quantity of expensive slurry.
The document GB 1410634 B discloses a thixotropic slurry which requires agitation for the fibers to remain evenly dispersed.
Suspension aids have been added to investment casting slurries in order to prevent the separation of dense and less dense components. One of the disadvantages of most suspension aids is that their addition to a slurry causes an increase in drying time of the coats made with that slurry. There is thus a tendency for a coat of the slurry to slump under the influence of gravity as it dries. The increased drying time for each coat can lead to a much longer and accordingly more expensive process.
The use of stable or calcined ceramics in binders and slurries is universal in the practice of investment casting. Such stable ceramic materials include calcined clays, artificially produced aluminas, mullites, naturally occurring zircons and fused silica. The stability of calcined materials arises due to the heat treatment process which they have undergone. This heat treatment is conducted at temperatures which are below the melting point of the material in question. The temperatures are however sufficient to cause the material to thermally decompose. This decomposition reaction involves the driving off of volatile fractions of the material. In the case of limestone calcination, the limestone (calcium carbonate) is heated to around 850° C. This heating drives off the volatile fraction of carbon dioxide gas leaving a more thermodynamically stable calcium oxide solid. This calcium oxide is the stable calcined product of the reaction. In other calcining processes, the heat treatment may bring about a phase transition from a less thermodynamically stable solid phase to a more thermodynamically stable solid phase. The skilled person would know that many solids such as ceramics, refractories and ores are capable of undergoing calcination processes. Uncalcined ceramics such as clays are not used in investment casting compositions since they would be expected to lead to shrinkage during the drying and heating of slurry coats due to loss of water.